1. Field of the Invention
The present invention pertains to a silicon nitride (Si.sub.3 N.sub.4) based sintered material which has high toughness and strength, and is excellent in antifracture characteristics, so that it can be suitably used to manufacture cutting tools to be employed under severe cutting conditions, and further relates to a process specifically adapted to manufacture this Si.sub.3 N.sub.4 based sintered material.
2. Prior Art
As disclosed in Japanese Patent Application B-Publication No. 60-20346, there is known an Si.sub.3 N.sub.4 based sintered material which is manufactured by preparing powders of Si.sub.3 N.sub.4, zirconium oxide (ZrO.sub.2), and magnesium oxide (MgO) as starting materials; blending these powders into a prescribed composition; compacting the mixture into a green compact; and sintering the green compact at a temperature of 1,600.degree. C. to 2,100.degree. C. under a pressure of 1.5 atm to 130 atm in a nitrogen atmosphere or an atmosphere of a mixture of nitrogen and inert gases.
This prior art Si.sub.3 N.sub.4 based sintered material exhibits great strength, but its toughness is not sufficient. Therefore, when the material is, for example, employed to manufacture cutting tools for high-speed continuous wet cutting or rough milling of cast iron, the cutting edge is susceptible to fracturing or chipping, resulting in unduly short tool life.